Wireless telecommunications systems sometimes are divided into a series of cell areas covering a service area. Each cell area has a transmitting base station using an operating frequency set comprising a plurality of radio channels to communicate with mobile subscribers. Each channel represents an information signal at a particular frequency carrier or band.
In many instances it is advantageous to combine these channels for transmission purposes. The channels can all be combined by a broadband signal combiner into a multi-subchannel signal at low power levels and then amplified by a single linear amplifier (or its equivalent, a plurality of linear amplifiers in parallel, each amplifying a reduced power version of the same multi-subchannel signal) to raise the multi-subchannel signal to an appropriate transmit power level.
Peak to average ratio compression is a technique of improving average power levels from a transmitter with limited peak power capability. Traditionally, this has been accomplished with adaptive gain control and clipping. These techniques create frequency domain splatter and increase the noise floor of the transmitted signal. In addition, such techniques are not very useful for amplitude modulated signals, which require a high degree of linearity. Furthermore, different types of signals can tolerate different amounts of distortion and thus different amounts of compression. Prior-art techniques have not attempted to tailor symbols on multiple subchannels, including empty subchannels, in a controlled, individualized manner to improve the peak to average power ratio.
Clearly then, a need exists for an improved method and apparatus for peak limiting in a modulator. Preferably, the method and apparatus will allow control of the compression in accordance with the type of information being sent and will tailor symbols on multiple subchannels, including empty subchannels, in a controlled, individualized manner to improve the peak to average power ratio.